We still masturbate routinely together.
My sister and I have had a long history of masturbating and touching each other together. It started when we were younger. We were both swimmers and one day while at a parent's friend's pool (alone, our parents and their friend were inside) and I asked her if I could touch her 'there.' She said yes. Many times after that we did the same thing, each touching each other...it was natural brother/sister curiosity. Eventually our parents discovered what was going on and made us stop. They just assumed that it was normal for all brothers/sisters to go through.
When I was a little older, around 13-14 and discovered masturbation, I remember thinking what would it be like to masturbate with her. We'd play numerous games together and one day I asked her to play doctor. I gave her an exam and spent a particularly long time 'examining' her vagina and breasts. I remember having huge boners during and amazing orgasms right after our doctor games. Finally she was the doctor, she was a little hesitant to see my penis but eventually I convinced her. She didn't masturbate it yet, but she did touch it, and explore it. I distinctly remember her commenting on the pubic hair that was developing. She was two years younger than me. When she was thirteen, I remember us having our first orgasm together. She was lying on her bed face down and I was rubbing her vagina from behind. I hadn't fingered a girl yet so I didn't really know what I was doing at this point. Eventually I did something right and I remember her asking me to 'keep doing that because it felt good.' I was lying on the bed next to her, dry humping the bed as I rubbed her. We both had an orgasm at the same time and it was amazing. I remember being embarrassed at how my boxers were soaked (she wouldn't let me be naked in bed with her). A few more glorious times we masturbated together. At one point each of us were totally naked. I finally convinced her to jack me off. Our best session at this age was in my room. She was naked on my bed, and I was naked in front of her. She was jacking me off and I was fingering her and touching her breasts. I cummed all over her and she freaked out. We stopped for about a year after that.
When she was a freshman in high school and I was a junior we were home on a random day off that our parents didn't have. I accidently (really it was an accident) walked into her room, asking her if she was ready to leave. She was topless and in very sexy underwear. It had been over a year and a half since I'd seen her naked. Her breasts had really developed and I really wanted to see what her vagina looked like. I got a boner instantly. She covered herself up, but the thoughts of before must have been going through both of our minds. I asked her if she'd like 'to do it' again. She said yes, but only if I 'did it' to her, and I kept my clothes on. We never called what we did together masturbating or masturbation. It was just called 'doing something.' She kept the towel over her breasts and slowly removed her underwear. Her vagina had really developed. She had a batch of pubic hair and had a gorgeous vagina in general. She lay on her bed with her legs open. I fingered her and even went inside but she asked me not to do that. We both orgasmed.
That was the start of our mutual masturbation. Usually, I will instigate out sessions (we call it masturbation now) but sometimes she will. We're both older now (21 and 19) and know what we like. She likes me to play with her breasts and flick her clit outside her underwear (I make her put on thongs) and this gets her very, very wet. We tend to masturbate together when no one is home, but sometimes we will do it late at night, if we're both horny.
I will usually be on her bed, in my boxers with my dick poking out. She will start in her bra and panties and then go to nothing. She shaves her vagina so there's thin layer of pubic hair and also enjoys me playing with her pussy lips/ pubic hair. Sometimes she will jerk me off (in a 69 position) but usually I will take care of myself after she's done. Our record is five orgasms for her and two for me in one session. She has clitoral orgasms and occasionally I will go in. I've never made her squirt, but her pussy is soaking wet all the while I masturbate her. A few times I've played with her butthole and a few other times we've gone further than just masturbating, but never sex. Both of us have had relationships, but we always do this. I love masturbating with her. It's great and we have a perfectly healthy relationship. It's just 'our thing.'